tailcat: keep netns on for macOS so a co-located VPN is bypassed

pickregion.go

@@ -13,7 +13,6 @@ import (
 
 	"tailscale.com/net/netcheck"
 	"tailscale.com/net/netmon"
-	"tailscale.com/net/netns"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 )
@@ -23,14 +22,10 @@ import (
 // nil error) if the netcheck report contained no usable region
 // latencies.
 func PickBestRegion(ctx context.Context, dm *tailcfg.DERPMap) (regionID tailcfg.DERPRegionID, err error) {
-	// Disable netns before netcheck binds its STUN sockets, not just
-	// at engine creation as createEngine does, which runs after this.
-	// tailcat has no TUN device whose routes netns would need to
-	// avoid, and netns's unprivileged fallback (SO_BINDTODEVICE to
-	// the default interface) blackholes loopback UDP, making every
-	// netcheck against a loopback DERP map burn the full 3-second
-	// STUN timeout before falling back to HTTPS probes.
-	netns.SetEnabled(false)
+	// Set the netns policy before netcheck binds its STUN sockets, not
+	// just at engine creation as createEngine does, which runs after
+	// this. See configureNetNS for why it depends on the OS.
+	configureNetNS()
 	nc := &netcheck.Client{
 		NetMon:  netmon.NewStatic(),
 		Verbose: Verbose,

tailcat.go

@@ -1905,7 +1905,7 @@ func createEngine(logf logger.Logf, lb *locoBackend) (err error) {
 	// call-me-maybe messages that advertise our UDP endpoints (see
 	// locoBackend.advertiseEndpoints).
 	conf.ForceDiscoKey = discoPrivateForNode(lb.priv)
-	netns.SetEnabled(false)
+	configureNetNS()
 	e, err := wgengine.NewUserspaceEngine(logf, conf)
 	if err != nil {
 		logf("wgengine.NewUserspaceEngine(tun %q) error: %v", "userspace-networking", err)
@@ -1916,6 +1916,26 @@ func createEngine(logf logger.Logf, lb *locoBackend) (err error) {
 	return nil
 }
 
+// configureNetNS sets the process-wide netns policy before tailcat opens
+// any sockets.
+//
+// On macOS netns stays on. It binds tailcat's UDP sockets and DERP
+// connections to the default physical interface, as tailscaled does, so
+// a co-located VPN that takes the default route, such as the Tailscale
+// app with an exit node selected, does not carry tailcat's WireGuard
+// packets inside its own tunnel. Loopback traffic still works from
+// sockets bound this way.
+//
+// Other platforms turn netns off. On Linux, netns's unprivileged fallback
+// (SO_BINDTODEVICE to the default interface) blackholes loopback UDP,
+// making every netcheck against a loopback DERP map burn the full
+// 3-second STUN timeout before falling back to HTTPS probes.
+func configureNetNS() {
+	if runtime.GOOS != "darwin" {
+		netns.SetEnabled(false)
+	}
+}
+
 // Client connects to a [Server] over a WireGuard tunnel relayed through DERP.
 // Populate Server (the only required field, or use the [NewClient]
 // shorthand), then just dial: [Client.Dial], the DialTCP methods, and the

tailcat_test.go

@@ -16,6 +16,7 @@ import (
 	"net/http/httptest"
 	"net/netip"
 	"os"
+	"runtime"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -35,6 +36,8 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
 	"tailscale.com/envknob"
 	"tailscale.com/net/netcheck"
+	"tailscale.com/net/netmon"
+	"tailscale.com/net/netns"
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest/integration"
@@ -1682,3 +1685,21 @@ func TestPeerKey(t *testing.T) {
 		}
 	}
 }
+
+// TestNetNSPolicy checks the netns policy that configureNetNS sets when a
+// Server starts. On macOS netns stays on, so tailcat's sockets bind to the
+// default physical interface instead of following a co-located VPN's
+// default route. Elsewhere it is off.
+func TestNetNSPolicy(t *testing.T) {
+	dm := integration.RunDERPAndSTUN(t, mkLogger(t, "derpstun"), "127.0.0.1")
+	s := &Server{Key: key.NewNode(), Logf: mkLogger(t, "server"), Region: dm.Regions[1]}
+	t.Cleanup(func() { s.Close() })
+	if err := s.Start(); err != nil {
+		t.Fatalf("server Start: %v", err)
+	}
+
+	bound := netns.Listener(t.Logf, netmon.NewStatic()).Control != nil
+	if want := runtime.GOOS == "darwin"; bound != want {
+		t.Errorf("on %s after Server.Start, netns binds sockets = %v; want %v", runtime.GOOS, bound, want)
+	}
+}